This invention relates to an exhaust silencing apparatus for outboard motors and other marine propulsion apparatus.
Outboard motors are generally constructed with an internal combustion engine as the power head. Various exhaust systems are employed for the engine, some of which provide tuning for maximum power at wide open throttle conditions. One highly satisfactory system channels the exhaust through the propeller hub. In such constructions, exhaust pipes or passageways are projected downwardly through the drive shaft housing and terminate in a passage extending downwardly through the propeller unit for exhaust below the water level. A typical system is illustrated in U.S. Pat. No. 3,808,807. When the engine is idling, however, the lower end of the exhaust discharge passageway is submerged within the water, creating excessive back pressure conditions. Generally, a pressure release system is provided by allowing the exhaust gases to escape upwardly through the water into the drive shaft housing and then outwardly via an idle relief exhaust passageway provided in the upper portion of the drive shaft housing and terminating above the idle water line. Although such systems provide a satisfactory solution to the back pressure problem, such exhaust passageways have been a source of undesired and considerable noise under idle speed conditions.